Method for fabricating a semiconductor structure having a crystalline alkaline earth metal oxide interface with silicon

ABSTRACT

A method for fabricating a semiconductor structure comprises the steps of providing a silicon substrate ( 10 ) having a surface ( 12 ); forming on the surface of the silicon substrate an interface ( 14 ) comprising a single atomic layer of silicon, oxygen, and a metal; and forming one or more layers of a single crystal oxide ( 26 ) on the interface. The interface comprises an atomic layer of silicon, oxygen, and a metal in the form XSiO 2 , where X is a metal.

FIELD OF THE INVENTION

[0001] The present invention relates in general to a method forfabricating a semiconductor structure including a crystalline alkalineearth metal oxide interface between a silicon substrate and otheroxides, and more particularly to a method for fabricating an interfaceincluding an atomic layer of an alkaline earth metal, silicon, andoxygen.

BACKGROUND OF THE INVENTION

[0002] An ordered and stable silicon (Si) surface is most desirable forsubsequent epitaxial growth of single crystal thin films on silicon fornumerous device applications, e.g., ferroelectrics or high dielectricconstant oxides for non-volatile high density memory and logic devices.It is pivotal to establish an ordered transition layer on the Sisurface, especially for subsequent growth of single crystal oxides,e.g., perovskites.

[0003] Some reported growth of these oxides, such as BaO and BaTiO₃ onSi(100) was based on a BaSi₂ (cubic) template by depositing one fourthmonolayer of Ba on Si(100) using reactive epitaxy at temperaturesgreater than 850° C. See for example: R. McKee et al., Appl. Phys. Lett.59(7), pp 782-784 (Aug. 12, 1991); R. McKee et al., Appl. Phys. Lett.63(20), pp. 2818-2820 (Nov. 15, 1993); R. McKee et al., Mat. Res. Soc.Symp. Proc., Vol. 21, pp. 131-135 (1991); U.S. Pat. No. 5,225,031,issued Jul. 6, 1993, entitled “Process for Depositing an OxideEpitaxially onto a Silicon Substrate and Structures Prepared with theProcess”; and U.S. Pat. No. 5,482,003, issued Jan. 9, 1996, entitled“Process for Depositing Epitaxial Alkaline Earth Oxide onto a Substrateand Structures Prepared with the Process”. However, atomic levelsimulation of this proposed structure indicates that it likely is notstable at elevated temperatures.

[0004] Growth of SrTiO₃ on silicon (100) using an SrO buffer layer hasbeen accomplished. T. Tambo et al., Jpn. J. Appl. Phys., Vol. 37 (1998),pp. 4454-4459. However, the SrO buffer layer was thick (100 Å), therebylimiting application for transistor films, and crystallinity was notmaintained throughout the growth.

[0005] Furthermore, SrTiO₃ has been grown on silicon using thick metaloxide buffer layers (60-120 Å) of Sr or Ti. B. K. Moon et al., Jpn. J.Appl. Phys., Vol. 33 (1994), pp. 1472-1477. These thick buffer layerswould limit the application for transistors.

[0006] Therefore, a method for fabricating a thin, stable crystallineinterface with silicon is needed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIGS. 1-2 illustrate a cross-sectional view of a cleansemiconductor substrate having an interface formed thereon in accordancewith the present invention;

[0008] FIGS. 3-6 illustrate a cross-sectional view of a semiconductorsubstrate having an interface formed from a silicon dioxide layer inaccordance with the present invention; and

[0009] FIGS. 7-8 illustrate a cross-sectional view of analkaline-earth-metal oxide layer formed on the structures illustrated inFIGS. 1-6 in accordance with the present invention.

[0010] FIGS. 9-12 illustrate a cross-sectional view of a perovskiteformed on the structures of FIGS. 1-8 in accordance with the presentinvention.

[0011]FIG. 13 illustrates a side view of the atomic structure of oneembodiment of the layers of FIG. 12 in accordance with the presentinvention.

[0012]FIG. 14 illustrates a top view along view line AA of FIG. 13 ofthe interface.

[0013]FIG. 15 illustrates a top view along view line AA of FIG. 13including the interface and the adjacent atomic layer of the substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] To form the novel interface between a silicon (Si) substrate andone or more layers of a single crystal oxide, various approaches may beused. Several examples will be provided for both starting with a Sisubstrate having a clean surface, and a Si substrate having silicondioxide (SiO₂) on the surface. SiO₂ is amorphous rather than singlecrystalline and it is desirable for purposes of growing additionalsingle crystal material on the substrate that a single crystal oxide beprovided as the interface.

[0015] Turning now to the drawings in which like elements are designatedwith like numbers throughout, FIGS. 1 and 2 illustrate a semiconductorstructure including a Si substrate 10 having a clean surface 12. A clean(2×1) surface 12 may be obtained with any conventional cleaningprocedure, for example, with thermal desorption of SiO₂ at a temperaturegreater than or equal to 850° C., or by removal of the hydrogen from ahydrogen terminated Si(1×1) surface at a temperature greater than orequal to 300° C. in an ultra high vacuum. Hydrogen termination is a wellknown process in which hydrogen is loosely bonded to dangling bonds ofthe silicon atoms at surface 12 to complete the crystalline structure.The interface 14 of a crystaline material may be formed by supplying (asshown by the arrows in FIG. 1) controlled amounts of a metal, Si, andO₂, either simultaneously or sequentially to the surface 12 at atemperature less than or equal to 900° C. in a growth chamber with O₂partial pressure less than or equal to 1×10⁻⁹ mBar. The metal applied tothe surface 12 to form the interface 14 may be any metal, but in thepreferred embodiment comprises an alkaline-earth-metal, such as barium(Ba) or strontium (Sr).

[0016] As the application of the Ba, Si, and O₂ form BaSiO₂ as theinterface 14, the growth is monitored using Reflection High EnergyElectron Diffraction (RHEED) techniques which are well documented in theart and which can be used in situ, i.e., while performing the exposingstep within the growth chamber. The RHEED techniques are used to detector sense surface crystalline structures and in the present processchange rapidly to strong and sharp streaks by the forming of an atomiclayer of the BaSiO₂. It will of course be understood that once aspecific manufacturing process is provided and followed, it may not benecessary to perform the RHEED techniques on every substrate.

[0017] The novel atomic structure of the interface 14 will be describedin subsequent paragraphs.

[0018] It should be understood by those skilled in the art that thetemperatures and pressures given for these processes are recommended forthe particular embodiment described, but the invention is not limited toa particular temperature or pressure range.

[0019] Referring to FIGS. 3-6, another approach comprises forming a Sisubstrate 10 having a surface 12, and a layer 16 of SiO₂ thereupon. Thelayer 16 of SiO₂ naturally exists (native oxide) once the Si substrate10 is exposed to air (oxygen) or it may be formed purposely in acontrolled fashion well known in the art, e.g., thermally by applying(arrows) oxygen onto the surface 12. The novel interface 14 may beformed at least in one of the two suggested embodiments as follows: Byapplying an alkaline-earth-metal to the surface 18 of SiO₂ layer 16 at700-900° C., under an ultra high vacuum. More specifically, the Sisubstrate 10 and the amorphous SiO₂ layer 16 are heated to a temperaturebelow the sublimation temperature of the SiO₂ layer 16 (generally below900° C.). This can be accomplished in a molecular beam epitaxy chamberor Si substrate 10 can be at least partially heated in a preparationchamber after which it can be transferred to the growth chamber and theheating completed. Once the Si substrate 10 is properly heated and thepressure in the growth chamber has been reduced appropriately, thesurface 12 of the Si substrate 10 having SiO₂ layer 16 thereon isexposed to a beam of metal, preferrably an alkaline-earth-metal, asillustrated in FIG. 5. In a preferred embodiment, the beam is Ba or Srwhich is generated by resistively heating effusion cells or from e-beamevaporation sources. In a specific example, Si substrate 10 and SiO₂layer 16 are exposed to a beam of Ba. The Ba joins the SiO₂ and convertsthe SiO₂ layer 16 into the interface 14 comprising BaSiO₂in acrystalline form. Alternatively, an alkaline-earth-metal may be providedto the surface 18 at lower temperatures, annealing the result at700-900° C., in an ultra high vacuum.

[0020] Once the interface 14 is formed, one or more layers of a singlecrystal oxide may be formed on the surface of the interface 14. However,an optional layer of an alkaline-earth-metal oxide, such as BaO or SrO,may be placed between the interface 14 and the single crystal oxide.This alkaline-earth-metal oxide provides a low dielectric constant(advantageous for certain uses such as memory cells) and also preventsoxygen from migrating from the single crystal oxide to the Si substrate10.

[0021] Referring to FIGS. 7 and 8, the formation of alkaline-earth-metaloxide layer 22 may be accomplished by either the simultaneous oralternating supply to the surface 20 of the interface 14 of analkaline-earth-metal and oxygen at less than or equal to 700° C. andunder O₂ partial pressure less than or equal to 1×10⁻⁵ mBar. Thisalkaline-earth-metal oxide layer 22 may, for example, comprise athickness of 50-500 Å.

[0022] Referring to FIGS. 9-12, a single crystal oxide layer 26, such asan alkaline-earth-metal perovskite, may be formed on either the surface20 of the interface 14 or the surface 24 of the alkaline-earth-metaloxide layer 22 by either the simultaneous or alternating supply of analkaline-earth-metal oxide, oxygen, and a transition metal, such astitanium, at less than or equal to 700° C. under an oxygen partialpressure less than or equal to 1×10⁻⁵ mBar. This single crystal oxidelayer 26 may, for example, comprise a thickness of 50-1000 Å and will besubstantially lattice matched with the underlying interface 14 oralkaline-earth-metal oxide layer 22. It should be understood that thesingle crystal oxide layer 26 may comprises one or more layers in otherembodiments.

[0023] Referring to FIG. 13, a side view (looking in the <{overscore(l)}10> direction) of the atomic configuration of the Si substrate 10,interface 14, and alkaline-earth-metal metal oxygen layer 26 is shown.The configuration shown comprises, in relative sizes, for illustrativepurposes, from larger to smaller, barium atoms 30, silicon atoms 32,oxygen atoms 34, and titanium atoms 36. The Si substrate 10 comprisesonly silicon atoms 32. The interface 14 comprises metal atoms (which inthe preferred embodiment are illustrated as barium atoms 30), siliconatoms 32, and oxygen atoms 34. The alkaline-earth-metal metal oxygenlayer 26 comprises barium atoms 30, oxygen atoms 34, and titanium atoms36.

[0024] Referring to FIG. 14, a top view of the interface along view lineAA of FIG. 13, shows the arrangement of the barium, silicon, and oxygenatoms 30, 32, 34.

[0025] Referring to FIG. 15, a top view along line AA of FIG. 13, showsthe interface 14 and the top atomic layer 11 of the Si substrate 10.

[0026] For this discussion, a monolayer equals 6.8×10¹⁴ atoms/cm² and anatomic layer is one atom thick. It is seen that the interface 14 shownin the FIGS. comprises a single atomic layer, but could be more than oneatomic layer, while the Si substrate 10 and the alkaline-earth-metalmetal oxide layer may be many atomic layers. Note that in FIG. 13, onlyfour atomic layers of the Si substrate 10 and only three atomic layersof the alkaline-earth-metal metal oxide layer 26 are shown. Theinterface 14 comprises a half monolayer of the alkaline-earth-metal, ahalf monolayer of silicon, and a monolayer of oxygen. Each barium atom30 is substantially equally spaced from four of the silicon atoms 32 inthe Si substrate 10. The silicon atoms 32 in the interface 14 aresubstantially on a line and equally spaced between thealkaline-earth-metal atoms in the <110> direction. Each silicon atom 32in the top layer of atoms in the Si substrate 10 is bonded to an oxygenatom 34 in the interface 14 and each silicon atom 32 in the interface 14is bonded to two oxygen atoms 34 in the interface 14. The interface 14comprises rows of barium, silicon, and oxygen atoms 30, 32, 34 in a 2×1configuration on a (001) surface of the Si substrate 10, 1× in the<{overscore (l)}10> direction and 2× in the <110> direction. Theinterface 14 has a 2×1 reconstruction.

[0027] A method for fabricating a thin, crystalline interface 14 withsilicon 10 has been described herein. The interface 14 may comprise asingle atomic layer. Better transistor applications are achieved by theinterface 14 being thin, in that the electrical coupling of theoverlying oxide layers to the Si substrate 10 is not compromised, and inthat the interface 14 is more stable since the atoms will more likelymaintain their crystalinity in processing.

What is claimed is:
 1. A method of fabricating a semiconductor structurecomprising the steps of: providing a silicon substrate having a surface;forming on the surface of the silicon substrate an interface comprisinga single atomic layer of silicon, oxygen, and a metal; and forming oneor more layers of a single crystal oxide on the interface.
 2. The methodof fabricating a semiconductor structure of claim 1 wherein the formingthe interface step includes forming a 2×1 reconstruction.
 3. The methodof fabricating a semiconductor structure of claim 1 wherein the formingan interface step includes forming the interface in an ultra-high-vacuumsystem.
 4. The method of fabricating a semiconductor structure of claim1 wherein the forming an interface step includes forming the interfacein a chemical vapor deposition system.
 5. The method of fabricating asemiconductor structure of claim 1 wherein the forming an interface stepincludes forming the interface in a physical vapor deposition system. 6.The method of fabricating a semiconductor structure of claim 1 whereinthe forming an interface step comprises forming a single atomic layer ofsilicon, oxygen, and an alkaline-earth-metal.
 7. The method offabricating a semiconductor structure of claim 6 wherein thealkaline-earth-metal is selected from the group of barium and strontium.8. The method of fabricating a semiconductor structure of claim 1wherein forming an interface step comprises the steps of: forming a halfof a monolayer of an alkaline-earth-metal; forming a half of a monolayerof silicon; and forming a monolayer of oxygen.
 9. A method offabricating a semiconductor structure comprising the steps of: providinga silicon substrate having a surface; forming amorphous silicon dioxideon the surface of the silicon substrate; providing analkaline-earth-metal on the amorphous silicon dioxide; and heating thesemiconductor structure to form an interface comprising a single atomiclayer adjacent the surface of the silicon substrate.
 10. The method offabricating a semiconductor structure of claim 9 wherein the heatingstep includes forming the interface with a 2×1 reconstruction.
 11. Themethod of fabricating a semiconductor structure of claim 9 wherein thesteps of providing an alkaline-earth-metal and heating the semiconductorstructure are accomplished in an ultra-high-vacuum system.
 12. Themethod of fabricating a semiconductor structure of claim 9 wherein thesteps of providing an alkaline-earth-metal and heating the semiconductorstructure are accomplished in a chemical vapor deposition system. 13.The method of fabricating a semiconductor structure of claim 9 whereinthe steps of providing an alkaline-earth-metal and heating thesemiconductor structure are accomplished in a physical vapor depositionsystem.
 14. The method of fabricating a semiconductor structure of claim9 wherein the heating step includes forming an interface having a singleatomic layer of silicon, oxygen, and an alkaline-earth-metal.
 15. Themethod of fabricating a semiconductor structure of claim 14 wherein thealkaline-earth-metal is selected from the group of barium and strontium.16. The method of fabricating a semiconductor structure of claim 9wherein heating step includes forming an interface step comprises thesteps of: forming a half of a monolayer of an alkaline-earth-metal;forming a half of a monolayer of silicon; and forming a monolayer ofoxygen.
 17. A method of fabricating a semiconductor structure comprisingthe steps of: providing a silicon substrate having a surface; providingan alkaline-earth-metal on the surface of the silicon substrate; andproviding silicon and oxygen to form an interface comprising a singleatomic interface with the surface of the silicon substrate.
 18. Themethod of fabricating a semiconductor structure of claim 17 wherein theproviding silicon and oxygen step comprises forming an interface havinga 2×1 reconstruction.
 19. The method of fabricating a semiconductorstructure of claim 17 wherein the steps of providing analkaline-earth-metal and providing silicon and oxygen are accomplishedin an ultra-high-vacuum system.
 20. The method of fabricating asemiconductor structure of claim 17 wherein the steps of providing analkaline-earth-metal and providing silicon and oxygen are accomplishedin a chemical vapor deposition system.
 21. The method of fabricating asemiconductor structure of claim 17 wherein the steps of providing analkaline-earth-metal and providing silicon and oxygen are accomplishedin a physical vapor deposition system.
 22. The method of fabricating asemiconductor structure of claim 17 wherein the providing silicon andoxygen step comprises forming a single atomic layer of silicon, oxygen,and an alkaline-earth-metal.
 23. The method of fabricating asemiconductor structure of claim 22 wherein the alkaline-earth-metal isselected from the group of barium and strontium.
 24. The method offabricating a semiconductor structure of claim 17 wherein the providingsilicon and oxygen step comprises the steps of: forming a half of amonolayer of an alkaline-earth-metal; forming a half of a monolayer ofsilicon; and forming a monolayer of oxygen.